This invention relates to expanded or foamed starch products having improved water and/or humidity resistant properties which comprise a select combination of starch or flour modified with alkylene oxide and a starch ester having an intermediate DS of from about 1.0 to 1.8 or a flour ester having a DS of from about 0.3 to 1.1.
The ability to provide expanded products using starch material has recently taken on increased interest because of the biodegradable nature of such materials. This biodegradable attribute possessed by starch lends itself to the formation of products which are environmentally safe and friendly. The term "biodegradable" as used herein refers to the susceptibility of a substance to decomposition by living things (organisms/microorganisms) and/or natural environmental factors, e.g., the ability of compounds to be chemically broken down by bacteria, fungi, molds and yeast. Plastics used in packaging and other structural applications are not biodegradable.
Despite the known biodegradable characteristics of starch, it has not been easy to convert starch based materials into formed structured products because it lacks some of the physical properties needed, particularly under high humidity conditions.
The use of starch and derivatives thereof to form various shaped articles has been shown in a number of recent patent publications. U.S. Pat. Nos. 5,035,930 and 5,043,196 issued on Jul. 30, 1991 and Aug. 27, 1991, respectively, disclose foamed shaped products formed from high amylose starch and particularly alkylene oxide modified starch. U.S. Pat. No. 5,095,054 issued on Mar. 10, 1992, discloses shaped articles having dimensional stability and enhanced physical properties that are made from a thermoplastic polymer composition comprising a destructurized starch and a hydrophobic, water insoluble polymer or compound having one or more functional groups. Melt-molded articles formed of starch or a starch-derived macromolecular substance in combination with an oxyalkylene group-containing vinyl alcohol copolymer are disclosed in U.S. Pat. No. 5,158,810 issued Oct. 27, 1992. In U.S. Pat. No. 5,153,037 issued Oct. 6, 1992, biodegradable foamed shaped products formed of an expanded modified flour are shown while in WO 93/20110 published Oct. 14, 1993, shaped articles formed from a melt comprising high DS starch esters of greater than 1.5 and especially 1.8 to 2.9 DS are disclosed.
Notwithstanding the number of recent disclosures showing articles formed from starch, there still is the need for foamed or expanded starch articles which not only are biodegradable but have excellent water and/or humidity resistant properties.